1. Field of the Invention
The present invention relates generally to methods for preparing artificial dental crowns, bridges, and other restorations. The invention also encompasses compositions used in such methods.
2. Brief Description of the Related Art
A dental professional prepares an artificial crown to cover a tooth for many different reasons. In general, the crown is used to restore the anatomy, function, and aesthetics of part or all of the coronal portion of the natural tooth. For example, the crown may be needed to: 1) protect a weak tooth from breaking (the tooth may be weak as a result of decay or other disease); 2) hold together parts of a cracked tooth; 3) restore a tooth that is already broken or severely worn down; 4) cover and support a tooth with a large filling; 5) hold a dental bridge in place; 6) cover misshaped or severely discolored teeth; or 7) cover a dental implant.
In a conventional procedure for preparing a dental crown, the patient must make multiple visits to the dentist. In the first visit, the dentist examines and prepares the tooth that will receive the crown. First, the dentist may take an impression of the patient's dental anatomy including the tooth that will ultimately receive the crown. To make the impression, the dentist commonly uses a dispensing syringe to inject a paste-like material into an impression tray. Then, the dentist inserts the filled tray into the patient's mouth, and the patient bites down on the paste in the tray to form the impression. After the dentist has taken the impression, the patient is anesthetized, and the tooth, which will receive the crown, is prepared. First, the dentist may remove any dental caries from the tooth using a dental bur or other instrument. Then, the dentist performs “crown prep” work on the tooth by filing and grinding it to a “core” or “stump”. A high-speed or low-speed handpiece, which is equipped with a diamond bur, is used typically to grind the tooth and prepare the core. Next, the dentist takes a final impression of the prepared tooth.
In order that a more accurate impression can be made, the dentist may first use a gingival retraction device to retract the gingival tissue around the tooth that will receive the crown. Dentists commonly use gingival retraction cord, which is pressed into the gingival sulcus around the prepared tooth, with a hand instrument. As the gingival tissue is retracted, the margins of the prepared tooth are exposed. The dentist thus can take a more accurate and detailed impression. The impression of the prepared tooth is made using a paste-like material in the same manner as discussed above. After taking the impression of the prepared tooth, the dentist sends the impression to a dental laboratory, which makes the permanent crown.
During the first office visit, the dentist places a temporary crown on the prepared tooth to cover and protect the tooth, while the permanent crown is being fabricated. The temporary crown is made from a polymeric paste-like material such as an acrylic. More particularly, a polymerizable material, which is prepared from base and catalyst pastes, may be used to form the temporary crown. The base and catalyst pastes may be dispensed from a double-barrel syringe having a dispensing tip with a static mixer. The base and catalysts pastes are extruded through the static mixer and directly onto a pre-made impression or plastic matrix. Then, the impression or matrix containing the polymerizable (mixed base and catalyst) material is inserted into the patient's mouth. The dentist presses on the impression or matrix so that the polymerizable material is molded over the prepared tooth. Then, the impression or matrix containing the molded, partially-cured material is removed from the patient's mouth. The polymerizable material is fully cured by chemical-curing, light-curing, heat-curing, or other suitable process, and the temporary crown is formed. The temporary crown is then cemented to the tooth using a temporary dental cement.
Various temporary crown and bridge materials are available from companies that supply dental products. For example, Integrity™ A2 (Dentsply International) is a temporary crown and bridge material that is made from base and catalyst pastes mixed together in a 10:1 (base/catalyst) volume ratio. Protemp™ 3 Garant™ A2 (3M ESPE) is another temporary crown and bridge material that mixes base and catalyst pastes in a 10:1 volume ratio. Structur™ 2 SC A3 (Voco) and Temphase™ (Kerr) are commercially available cartridges that mix the base and catalyst pastes in a 1:1 volume ratio.
Compositions that can be used to make temporary crown and bridges are described generally in the patent literature. For example, May et al., U.S. Pat. No. 5,376,691 discloses dental cement for making temporary crowns and bridges. The dental cement is prepared from a first paste comprising a difunctional acrylate such as urethane diacrylate, an activator such as a tertiary amine, and a radiopaque filler such as barium and/or strontium glasses. The second paste includes no substances having active double bonds, a catalyst such as dibenzoyl peroxide, a silicon dioxide material, and a softener that cannot be polymerized along with the other components but is sufficiently insoluble in the mouth. The softeners can be selected from such compounds as liquid paraffins, long-chain glycols, and inert alkylphthalates.
Tateosian et al., U.S. Pat. No. 5,554,665 discloses a dental composition that is formed by the static mixing of two complementary pastes. A catalyst paste includes a polymerizable methacrylate, a peroxide, and a stabilizer such as butylated hydroxytoluene. The stabilizer is effective at preventing polymerization for at least 180 days at 23° C. A complementary accelerator and radiation-cure initiator paste includes a polymerizable methacrylate and a reducing agent for the peroxide such as dihydroxyethyl-p-toluidine. According to the '665 patent, the paste compositions preferably have substantially the same viscosity and are mixed in a volume ratio between 1:1 and 1:5.
Xie, U.S. Pat. No. 5,977,199 discloses a delivery system for delivering dental cement material for making temporary crowns and bridges. A catalyst paste and base paste are dispensed from a dual cartridge and mixed in a static mixer to form a polymerizing material. The catalyst paste comprises at least one polymerizable monomer, polymerization initiator, polymerization inhibitor, and filler. The base paste comprises at least one polymerizable monomer, at least one polymerization accelerator, polymerization inhibitor, and filler. According to the '199 patent, the viscosity of the catalyst paste must be substantially greater than the viscosity of the base paste in order for the mixture to cure effectively.
At the second office visit, the dentist removes the temporary crown. The dentist cleans the tooth, removing any residual temporary cement. The dentist further checks the color and occlusal fit of the permanent crown. If satisfactory, the dentist anesthetizes the tooth and then affixes the permanent crown to the tooth using permanent cement.
As described above, in a traditional crown preparation, fitting, and mounting procedure, the patient must make multiple visits to the dentist. Furthermore, the procedure involves the preparation of temporary or provisional crowns, which can be time-consuming and costly. The patient and dentist may feel added stress and anxiety due to the many anesthetization and fitting steps involved in this conventional process. It thus would be desirable to develop a method, whereby a dentist could prepare a permanent dental crown for a patient in a single office visit. Ideally, the dentist would be able to design and fabricate the crown “chair-side” and mount the crown on the patient's tooth in a single office visit. The crown should be made from a material having good aesthetics so that the crown matches the shade of the natural teeth. The crown should further have good mechanical strength and integrity so that the crown will not break or fracture easily.
The present invention provides such a method for making dental crowns and compositions that can be used to make such crowns. The method is efficient and allows the dentist to provide a crown that is aesthetically-pleasing and has good mechanical strength as well as other beneficial features and advantages.